Quaternary geology of the Fremont River drainage basin, Utah

The Fremont River drainage basin has a variety of geomorphic features that provide insight into the long-term landscape development of the catchment. 3He exposure ages from boulders from the most well preserved moraines in the Fish Creek drainage of Boulder Mountain indicate a last glacial maximum (LGM) advance ∼23.1 +/- 1.3 to 20.0 +/- 1.4 kyr ago and a later and smaller advance ∼16.8 +/- 0.5 to 15.2 +/- 0.5 kyr ago. Boulders exposed in moraines deposited from the western outlet glacier of the Boulder Mountain ice cap also indicate deposition during the peak LGM and range from 20.2 +/- 1.5 to 22.5 t 2.5 ka. No geomorphic or cosmogenic evidence of a glacial advance older than the LGM has yet been found on Boulder Mountain.; Mass movement deposits rich in volcanic boulders dominate much of the geomorphology of the central and eastern Fremont River drainage basin. Rotational slumps, landslides, debris-flows, and hyper concentrated flow run-out deposits rich in volcanic boulders mantle pediments and fluvial terraces cut into the local sedimentary bedrock in many parts of the drainage basin. Over time, the physical and chemical toughness of the volcanic clasts relative to the weaker local sedimentary bedrock leads to boulder armoring and topographic inversion. 3He exposure ages of multiple boulders from eleven of these armored and now abandoned surfaces range from ∼96 to 907 ka (minimum ages), and suggest local incision rates around the surfaces of 0.12 to 0.41 m/kyr (maximum rates). Several of the more distal armored surfaces are situated high above the modern landscape and likely pre-date much of the canyon cutting in the drainage basin. The exposure ages of the deposits that were sampled do not appear to show any relation to major periods of global glaciation. Boulder armoring likely rules out the possibility of topographic or geomorphic steady state in the Fremont River drainage basin. High rates of landscape change can be inferred from both the armored surface incision rate data and longer-term estimates of mean exhumation in the drainage basin. Much of this extremely fast incision and exhumation is likely related to the integration of the Colorado River system in the latest Miocene.